The present invention generally relates to electrophotographic copying machines and, more particularly, to a cleaning apparatus included in such a machine which applies a film-forming material of a small coefficient of friction onto a photosensitive element while removing a toner residual thereon by means of a cleaning blade.
An electrophotographic copying machine to which the present invention is applicable is of the type which forms a latent image electrostatically on a surface of a photosensitive element, processes the latent image with a toner powder into a visible toner image, transfers the toner image onto a sheet material to form a copy, and cleans the photosensitive element to remove residual particles of the toner therefrom so that the photosensitive element becomes prepared for another copying cycle.
In this type of copying machine, transfer of a powdery toner image from the photosensitive element to a sheet is under the influence of a relative humidity. Particularly, it is adversely affected by high relative humidities. When the relative humidity is high, an electrostatic force, Van der Waals force and other various influential forces cause a major part of the powder to constitute deposits adhered to the photosensitive element. This not only lowers the density of an image reproduced on a sheet but allows the toner image to remain on the photosensitive element. Therefore, complete image transfer and complete removal of the residual toner image in a copying cycle are key factors to the prevention of a ghost image during image transfer in the next copying cycle as well.
It is well known to clear residual toner particles from the surface of the photosensitive element using a cleaning blade. A cleaning effect attainable with a cleaning blade is excellent because the cleaning blade has its leading end or edge usually held in positive pressing contact with the photosensitive element to remove toner particles by intense friction. However, where use is made of a photosensitive element formed of a relatively soft material, the cleaning blade tends to damage the surface of the photosensitive element and/or cause wear of the same surface resulting in a short service life of the photosensitive element. An expedient to settle this problem may be the use of a cleaning blade formed of polyurethane rubber or like highly wear resistant material and a photosensitive element formed of a material which stands relatively intense friction. This expedient still fails to preclude incomplete cleaning due to wear of an edge portion at the leading end of the cleaning blade. Experiments showed that an expected cleaning effect becomes unattainable when the edge portion of such a cleaning blade wears by 20-50 microns.
Japanese Patent Publication No. 51-22380/1976 for instance discloses a method designed to eliminate such drawbacks inherent in the cleaning system of the type using a blade while promoting efficient transfer of a toner image from the photosensitive element to a sheet and efficient removal of a residual toner image by the blade. According to this method, a certain film-forming material having a small coefficient of friction is applied to a surface of the photosensitive element at the cleaning station so as to serve as a kind of lubricant. Zinc stearate is generally accepted as a film-forming material which gives a favorable result. Another example of such a material may be a metal salt which is dense, hydrophobic and with a stable fatty acid. Various kinds of dense and hydrophobic metal salts with stable fatty acids are stated in Japanese Patent Publication No. 51-22380/1967. A problem encountered here is that, the larger the amount of application of such a material onto the photosensitive drum, the greater the cleaning efficiency grows but, at the same time, the lower the image density becomes because the total amount of toner allowed to adhere to the photosensitive element during development is limited; the smaller the amount of the material, the poorer the cleaning efficiency though the higher the image density due to an increase in the total amount of toner adhesion during development.
It has been a common practice to apply a film-forming material of the type described either periodically or continuously, all in a fixed amount. With this mode of application, however, whether a current amount of the material supplied to the photosensitive element is proper cannot be known at all. If the amount of supply is short, the image density becomes excessive and, if the amount of supply is excessive, the image density becomes short.